Monocrystalline silicon is the most dominant material for photovoltaic applications. The high efficiencies associated with monocrystalline solar cells, combined with the abundance of material, garner appeal for continued use and advancement. But the high processing cost of crystalline silicon material limits the widespread use of these solar modules. At present, the cost of “wafering,” or crystallizing silicon and cutting a wafer, accounts for about 40% finished solar cell cost. If a more direct way of making wafers were possible, great headway could be made in lowering the cost of solar cells.
There are different known methods of growing monocrystalline silicon and releasing or transferring the grown wafer. Regardless of the method, a low cost epitaxial deposition process is assumed.
Silicon epitaxial (epi) deposition was originally developed for the semiconductor industry. The requirements for the semiconductor industry, in both film properties and cost, are nearly polar opposites of requirements in the solar field. For example, semiconductor epi films are typically less than 5 μm (1 μm=10−6 meter) thick, while solar requires 60-80 μm of silicon. In order to achieve economies in the solar industry, the silicon cost per watt must reside in the $0.25/watt or approximately $1.00/wafer (assuming a 4 watt cell).
The precursor chemistry for epi is predominantly trichlorosilane (TCS), although for thinner films silane (SiH4) may also be used. Epitaxial deposition for each chemical poses unique requirements and challenges in both equipment architecture and process conditions. Based on low cost and abundance, TCS is the chemistry of choice for the solar industry. The present disclosure will generally be described with regard to TCS, but one of ordinary skill in the art will recognize its applications to silane and other precursor chemicals.
In order to achieve the necessary economy for solar applications, process cost modeling is studied to identify and optimize equipment performance. Three categories of cost make up the total cost picture: fixed cost (FC), recurring cost (RC) and yield cost (YC). FC is made up of items such as equipment purchase price, installation cost, and robotics or automation cost. RC is largely made up of electricity, gases, chemicals, operator salaries, and maintenance technician support. YC may be interpreted as the total value of parts lost during production.
To achieve the cost of ownership (CoO) numbers required by the solar field, all aspects of the cost picture must be optimized. The qualities of a low-cost process are (in order of priority): 1) high productivity, 2) high yield, 3) low RC, and 4) low FC.
Designing highly productive equipment requires a good understanding of the process requirements and reflecting those requirements in the architecture. High yield requires a robust process and reliable equipment. Low RC turns out to be the dominant component in the cost model. RC can impact plant site selection based on, for example, cost of local power or availability of bulk chemicals. FC, although important, is diluted by equipment productivity.
With the above said, in summary, a highly productive, reliable, efficient reactor may be essential for the production of low cost solar cells.